1. Field of the Invention
The present invention relates to non-reducing dielectric ceramics, monolithic ceramic capacitors using the same, and methods for making the non-reducing dielectric ceramics.
2. Description of the Related Art
In various electronic devices, the rapid trends toward a reduction in size and greater packing density are producing an increasing demand for monolithic ceramic capacitors which allow such trends to continue to advance. Also, the use of the monolithic ceramic capacitors is being investigated in other industrial fields, including for use in vehicles and the like. Other desired requirements for the monolithic ceramic capacitors include further reduction in cost and higher reliability.
The need to meet these requirements has promoted the development of non-reducing dielectric ceramic materials which use inexpensive base metals as internal electrode materials, which are not changed into semiconductive materials during firing in a neutral or reducing atmosphere with a low oxygen partial pressure so as not to oxidize the internal electrode materials, and which exhibit superior dielectric characteristics.
For example, as non-reducing dielectric ceramic materials, Japanese Unexamined Patent Application Publication Nos. 60-131708, 63-126117, 5-217426, 10-335169, 10-330163, and 11-106259 disclose (Ca1-xSrx)m(Zr1-yTiy)O3-based, [(CaxSr1-x)O][(TiyZr1-y)O2]-based and (CaO)x(Zr1-yTiy)O2-based compositions.
The use of these non-reducing dielectric ceramic materials enables production of inexpensive, reliable monolithic ceramic capacitors which are not converted into semiconductive materials during firing in reducing atmospheres and which use base metals such as nickel and copper as internal electrodes.
In the non-reducing dielectric ceramics disclosed in Japanese Unexamined Patent Application Publication Nos. 60-131708 and 63-126117, the main component materials, e.g., calcium carbonate (CaCO3), strontium carbonate (SrCO3), titanium dioxide (TiO2) and zirconium dioxide (ZrO2), a subsidiary component material, e.g., manganese dioxide (MnO2), and a mineralizer, e.g., silicon dioxide (SiO2), are simultaneously calcined in order to prepare ceramics represented by (Ca1-xSrx)m(Zr1-yTiy)O3. The calcined raw material powder does not have a single perovskite structure, but, rather, has a mixed crystal system containing a perovskite primary crystal phase and other secondary crystal phases according to analysis by X-ray diffraction. Also, a dielectric ceramic obtained by firing this calcined raw material powder in a reducing atmosphere has a mixed crystal system. Such an non-homogeneous crystal structure in the dielectric ceramic tends to reduce the reliability of devices as the thickness of the ceramic is reduced to produce compact high-capacitance monolithic ceramic capacitors when they are subjected to high-temperature loading lifetime testing.
In the non-reducing dielectric ceramics disclosed in Japanese Unexamined Patent Application Publication Nos. 5-217426 and 10-335169, powders of calcium titanate (CaTiO3), strontium titanate (SrTiO3), strontium zirconate (SrZrO3) and calcium zirconate (CaZrO3) are used as starting materials in order to prepare ceramics represented by (Ca1-xSrx)m(Zr1-yTiy)O3 and [(CaxSr1-x)O][(TiyZr1-y)O2]. After weighing these powders, the resulting ceramic is obtained through wet mixing, molding, binder removal and firing. In this method, however, CaTiO3, SrTiO3, SrZrO3 and CaZrO3 having perovskite structures barely dissolve into each other. Therefore, the resulting dielectric ceramic has a mixed crystal system including a plurality of perovskite crystal phases. When the thickness of the elements is reduced to produce compact high-capacitance monolithic ceramic capacitors, the lifetimes of the monolithic ceramic capacitors in a high-temperature loading lifetime test vary and the reliability thereof tends to be impaired.
In the non-reducing dielectric ceramics disclosed in Japanese Unexamined Patent Application Publication Nos. 10-330163 and 11-106259, predetermined amounts of calcium carbonate (CaCO3), titanium dioxide (TiO2), zirconium dioxide (ZrO2) and manganese carbonate (MnCO3) are used as starting materials, a predetermined amount of glass component is used, and these are mixed, molded, subjected to binder removal and fired in order to prepare ceramics represented by (CaO)x(Zr1-yTiy)O2. In this method, however, the formation of a perovskite crystal phase as the primary crystal phase is impaired and the resulting dielectric ceramic has a mixed crystal system including the perovskite primary crystal phase and other secondary crystal phases. When the thickness of the elements is reduced to produce compact high-capacitance monolithic ceramic capacitors, the reliability of the monolithic ceramic capacitor tends to be reduced.
Accordingly, it is an object of the present invention to provide a non-reducing dielectric ceramic which does not cause deterioration of insulating resistance and dielectric loss during firing in a neutral or reducing atmosphere, and which exhibits a prolonged lifetime with a reduced variation in high-temperature loading lifetime testing when the thickness of the elements is reduced, and high reliability.
It is another object of the present invention to provide a monolithic ceramic capacitor using the non-reducing dielectric ceramic. It is still another object of the present invention to provide a method for making a non-reducing dielectric ceramic.
A non-reducing dielectric ceramic according to the present invention comprises Ca, Zr and Ti as metallic elements and does not contain Pb. In a CuKxcex1 X-ray diffraction pattern, the ratio of the maximum peak intensity of secondary crystal phases to the maximum peak intensity at 2xcex8=25xc2x0 to 35xc2x0 of a perovskite primary crystal phase is about 12% or less, wherein the secondary crystal phases include all the crystal phases other than the perovskite primary crystal phase. The ceramic may be represented by ApBO3 where A includes Ca, B includes Zr and Ti, and p is about 0.98 to 1.02.
The ratio of the maximum peak intensity of the secondary crystal phases to the maximum peak intensity of the perovskite primary crystal phase is preferably about 5% or less and more preferably about 3% or less.
Since the ratio of the maximum peak intensity of the secondary crystal phases to the maximum peak intensity of the perovskite primary crystal phase is about 12% or less, the secondary crystal phase content in the overall crystal phases is low. Thus, the resulting dielectric ceramic does not cause deterioration of insulating resistance and dielectric loss during firing in a neutral or reducing atmosphere, and exhibits a prolonged lifetime with a reduced variation in high-temperature loading lifetime testing when the thickness of the dielectric ceramic layer is reduced to about 5 mm or less and high reliability.
A monolithic ceramic capacitor in accordance with the present invention comprises a plurality of dielectric ceramic layers, internal electrodes provided between dielectric ceramic layers and external electrodes electrically connected to the internal electrodes, wherein the dielectric ceramic layers comprise the above-mentioned dielectric ceramic and the internal electrodes comprise a base metal. The base metal is preferably elemental nickel, a nickel alloy, elemental copper or a copper alloy.
Since the monolithic ceramic capacitor in accordance with the present invention uses the above-mentioned non-reducing dielectric ceramic, the monolithic ceramic capacitor does not cause deterioration of insulating resistance and dielectric loss during firing in a neutral or reducing atmosphere, and exhibits a prolonged lifetime with a reduced variation in high-temperature loading lifetime testing when the thickness of the dielectric ceramic layer is reduced to about 5 xcexcm or less, and high reliability.
In a method for making a non-reducing dielectric ceramic comprising Ca, Zr and Ti as metallic elements and not containing Pb, and in a CuKxcex1 X-ray diffraction pattern, the ratio of the maximum peak intensity of secondary crystal phases to the maximum peak intensity at 2xcex8=25 to 35xc2x0 of a perovskite primary crystal phase is about 12% or less, wherein the secondary crystal phases include all the crystal phases other than the perovskite primary crystal phase, the method comprises the steps of:
(A) calcining an uncalcined B-site component powder to prepare a calcined B-site component powder, wherein the dielectric ceramic is represented by the general formula ABO3;
(B) preparing a A-site component powder from A-site component materials;
(C) mixing the B-site component powder and the A-site component powder to prepare an uncalcined primary material powder;
(D) calcining the uncalcined primary material powder to prepare a calcined primary material powder;
(E) adding at least one of the A-site component powder and the B-site component powder to the calcined primary material powder for finely adjusting the composition of the calcined primary material powder to prepare a secondary material powder; and
(F) molding and firing the secondary material powder in a neutral or reducing atmosphere.
This method can produce dielectric ceramics with high reproducibility and high efficiency.